cook the onions in 2 tablespoons (30 ml) of the butter in a large soup pot over moderate heat until tender but not browned. Stir in the flour and cook for 2 minutes. Add the chicken broth, parsley, bay leaf, thyme, and mushroom stems and simmer covered for 30 minutes.

strain the soup through a fine strainer, pressing the mushroom stems to extract as much moisture as possible. Melt 2 tablespoons (30 ml) of the butter in a separate saucepan and cook the sliced mushroom caps, covered, over low heat for 5 minutes. Add the mushroom caps and their cooking juices to the strained broth and simmer for 10 minutes.

remove from the heat. Beat the egg yolks and the cream together in a small bowl. Add a little of the simmering broth to the egg mixture and stir well. Add the egg mixture to the soup and stir over moderate heat for a minute or two, being careful not to let the soup come to a boil..